Seat occupancy sensors are nowadays commonly used in automotive vehicles in order to improve the efficiency of secondary seat restraint systems, such as e.g. the vehicle airbags or the seat belt pretension systems. The seat occupancy sensors usually comprise a plurality of individual switching elements, which are arranged in an array configuration and associated to a seating surface of the vehicle seat. The individual switching elements are typically configured as pressure sensors, e.g. force sensing resistors, in which an electrical resistance is depending on the force acting on the seat.
In a possible application of those seat occupancy sensors, an actual seat occupancy status is determined by means of the seat occupancy sensors and, in case of a car crash situation, the airbags associated with the respective seat are deployed only if the actual seat occupancy status requires such deployment. In a different application, the signal of the seat occupancy sensors are used in a seat belt warning system for generating a warning signal if a specific vehicle seat is occupied and the corresponding seat belt is not fastened.
One issue with those seat sensor systems is related to the temperature dependent behavior of the switching elements and/or the environment in which the switching elements are arranged. In fact due to the behavior of the seat material (trims, foams, and suspension and seat structure) and sensors in different temperatures, the sensing capability of sensors is affected. Often with colder temperature, the sensor and seat (i.e. the whole sensing system) will get more insensitive, whereas, at higher temperatures, the whole system can get more sensitive and as a result, the force/pressure behavior is changing (harder vs. softer).
This means that in order to reliably detect an occupancy of a vehicle seat in a low temperature range, it would be preferable to have rather sensitive sensors/sensor features (cells) to make sure to reliably detect the occupant. At a higher temperature range on the other hand, the high sensitivity of the sensor cells may cause the sensor to be triggered by undesired low weight objects such as a bag placed on the seat. It follows that for the high temperature range, it would be preferable to have less sensitive sensors/sensor features (cells) so as to avoid detection of undesired objects on the seat. In other words for low temperatures up to a temperature Tx, one would prefer a sensitive sensor/sensor cell, while above a temperature Tx to high temperatures one would prefer an insensitive sensor/sensor cell. In the environment of an automotive vehicle, the temperature ranges may e.g. from LT=−40° C. to Tx for the low temperature range and from Tx to HT=85° C. Depending on the material used for the seat/sensor, Tx may be approx. 45-65° C.
It will be noted that depending on the application, the sensitivity of a sensing system may decrease with decreasing temperature or increase with decreasing temperature. This means that depending on the application it may also be interesting to have an insensitive sensor/sensor cell for low temperatures up to a temperature Tx and a more sensitive sensor/sensor cell above a temperature Tx to high temperatures.